1. Field of the Invention
The present invention relates to a production method of an active material for a positive electrode of an alkaline secondary battery, a positive electrode using the produced active material, and a production method of an alkaline secondary battery using the positive electrode, more specifically to a production method of an active material for a positive electrode having high utilization as an active material, unsusceptible to deterioration even in a long term storage, a positive electrode using the active material, and a production method of an alkaline secondary battery having an excellent high ratio discharge characteristic and capable of restraining the decline of the discharge capacity even at the time of recharging after leaving for a long time in an over discharge state.
2. Prior Art
Typical examples of alkaline secondary batteries include a nickel-hydrogen secondary battery and a nickel-cadmium secondary battery. In these batteries, a nickel electrode mainly comprising nickel hydroxide as the positive electrode active material is assembled as the positive electrode.
As the nickel electrode, two kinds, that is, a sintered type and a non-sintered type are known.
Among them, a non-sintered type nickel electrode is produced as follows.
A viscous paste mixture for positive electrode is prepared by kneading a nickel hydroxide powder which functions as an active material for a positive electrode and a binder such as carboxy methyl cellulose, methyl cellulose, sodium polyacrylate, and polytetrafluoroethylene with water. Then, the paste is filled or applied to a collector such as a three-dimensional substrate of a foamed nickel substrate, a net-like sintered substrate made of metal fibers or a non-woven fabric with the surface applied with nickel plating, and a two-dimensional substrate of a nickel punching sheet and an expand nickel, followed by a drying treatment and a press forming so that the above-mentioned paste mixture is filled and supported in the collector in the dry state.
Since a non-sintered type nickel electrode produced by above mentioned method has a higher filling density of a nickel hydroxide (active material) compared with a sintered type, it is advantageous in that a battery with a high discharge capacity can be provided.
With the above-mentioned nickel electrode as the positive electrode, an alkaline secondary battery is assembled as follows.
A generating element is produced by placing a separator having the electric insulating property and the liquid maintaining property between the above-mentioned nickel electrode and a predetermined negative electrode.
When the purposed alkaline secondary battery to be produced is a nickel-cadmium secondary battery, a negative electrode supporting a mixture for negative electrode having a cadmium compound such as a metal cadmium and a cadmium hydroxide as the negative electrode active material is used. When the purposed alkaline secondary battery to be produced is a nickel-hydrogen secondary battery, a negative electrode supporting a mixture for negative electrode mainly comprising a hydrogen absorbing alloy is used. As a separator, a non-woven fabric of a polyamide fiber or a non-woven fabric of a polyolefin fiber such as a polyethylene fiber and a polypropylene fiber, applied with a hydrophilic treatment can be used commonly.
The above-mentioned generating element is placed in a battery can with the bottom also serving as a negative terminal comprising a nickel plating steel plate, for example, and a predetermined amount of an alkaline electrolyte is filled therein. Examples of the alkaline electrolyte include, in general, an aqueous solution of sodium hydroxide, an aqueous solution of potassium hydroxide, an aqueous solution of lithium hydroxide, and an optional mixture thereof.
Then, after placing a positive electrode terminal at the opening of the battery can, the entirety is sealed so as to provide a battery.
The initial charging is conducted to the assembled battery so as to apply the activating treatment for the nickel hydroxide, which is the activate material, prior to the shipment. In general, the initial charging is conducted under the condition where quantity of electricity more than 100% of the theoretical capacity of the assembled nickel electrode can be charged.
In the above-mentioned nickel electrode, it is important to improve the conductivity within the active material (nickel hydroxide), and between the active material and the collector for improving the utilization of the active material.
In order to achieve the task, the following treatment has been adopted conventionally.
In preparing a paste mixture for a positive electrode, a predetermined amount of particles of a metal cobalt, a cobalt compound such as cobalt hydroxide, cobalt trioxide, cobalt tetroxide, and cobalt monoxide, or a mixture thereof are added as a conducting material so as to produce a powdery material mixed with the nickel hydroxide particles by a predetermined ratio to be used as the active material.
If a nickel electrode supporting the active material produced by above mentioned method is assembled in an alkaline secondary battery as the positive electrode, the metal cobalt or the cobalt compound contained in the above-mentioned powdery material is dissolved temporarily in the alkaline electrolysis solution as complex ions, and is distributed on the surface of the nickel hydroxide particles, which are the active material. At the time of the initial charging of the battery, the complex ions are oxidized earlier than the nickel hydroxide so as to be converted to an oxide of higher order such as oxycobalt hydroxide. It is precipitated among the nickel hydroxide particles, which are the active material, and between the active material layer and the collector so as to form a conductive matrix. Therefore, the collecting efficiency at the nickel electrode can be improved, and consequently, the utilization of the active material can be improved. In that case, with a larger number of contacting points between the above-mentioned conductive matrix and the nickel hydroxide particles, the utilization of the nickel hydroxide particles (active material) can further be improved.
Moreover, a method of treating the powdery material produced as mentioned above with a heat alkaline aqueous solution is also known.
Specifically, the above-mentioned powdery material is soaked in an alkaline aqueous solution so that the alkaline aqueous solution is applied or impregnated to the powdery material. Then, the entirety is filtrated so that the substance obtained by the filtration is heated at a predetermined temperature. According to the method, a part of the metal cobalt or the cobalt compound contained in the powdery material is dissolved in the heat alkaline aqueous solution as complex ions so that they homogeneously cover the surface of the particles mainly comprising the nickel hydroxide. Then, it is changed into an active material having factors of forming the above-mentioned conductive matrix.
However, in this method of treating the above-mentioned powdery material with the heat alkaline aqueous solution, the cobalt complex ions temporarily dissolved in the heat alkaline aqueous solution may be precipitated again in the cooling process of the heat alkaline aqueous solution so as to form particle aggregates by bonding adjacent particles with each other. Therefore, since the obtained treated substance is an assembly of the above-mentioned particle aggregates, if paste mixture for a positive electrode is prepared with the treated substance, the distribution state of the nickel hydroxide particles, which are the positive electrode active material in the paste, becomes uneven, and thus it is problematic in that the effective use as the active material cannot be realized.
As another method, for example, a method of using a powdery material prepared by introducing particle mainly comprising nickel hydroxide into an alkaline aqueous solution controlled to be in the range of pH 11 to 13 and gradually adding, for example, an aqueous solution of cobalt sulfate so as to cover the surface of the above-mentioned particles with the formed cobalt compound like cobalt hydroxide for the use as the active material can be presented.
According to the method, the surface of the nickel hydroxide particles can be covered with a small amount of a cobalt compound, however, a problem is involved in that the formation amount of the above-mentioned conductive matrix is reduced accordingly.
In either case, in order to improve the utilization of the active material, it is advantageous to make a larger amount of the conductive matrix by increasing the amount of the metal cobalt or the cobalt compound in the above-mentioned powdery material.
However, with a larger amount of the metal cobalt or the cobalt compound is included in the above-mentioned powdery material for improving the utilization of the active material, not only does the production cost of the nickel electrode increase, but also the relative ratio of the nickel hydroxide particles to serve as the positive electrode active material decreases, and thus it is disadvantageous for achieving a high capacity of the battery.
Taking the above-mentioned into the account, an active material capable of performing the effects even with a minimum amount of the metal cobalt or the cobalt compound is preferable as the active material.
In the case of a battery where a nickel electrode with a large amount of a metal cobalt or a cobalt compound is contained, the cobalt compound enters into the crystal structure of the nickel hydroxide particles under the over discharge state, and thus it is problematic in that the above-mentioned effects inherent to the cobalt compound, that is, the formation of the conductive matrix cannot proceed so as to disturb the improvement of the utilization of the active material.
Recently, with various kinds of electronic appliances provided in a portable form, a nickel-hydrogen secondary battery or a nickel-cadmium secondary battery to be used as the driving power source is strongly required not to cause the decline of the capacity even after leaving a long time, to show a high ratio discharge characteristic from the initial stage, and to show an excellent discharge characteristic even in a low temperature environment, in addition to have a high capacity.
In this case, in order to improve the high ratio discharge characteristic, it is known that a large amount of a metal cobalt or a cobalt compound in a nickel electrode, which is the positive electrode, is effective for improving the high ratio discharge characteristic.
However, these methods run counter to the above-mentioned achievement of the high capacity of the battery, and thus they are not a method for improving the large current discharge characteristic without sacrificing a high capacity.
On the other hand, the official gazettes of Japanese Patent Publication Laid-Open No. 8-195218 and 8-236145 disclose a method of conducting the initial charging in a high temperature until the metal cobalt or the cobalt compound included in the nickel electrode is oxidized completely into the oxycobalt hydroxide.
According to the method, since the metal cobalt or the cobalt compound included in the nickel electrode can contribute to the formation of the conductive matrix without loss, a high capacity of the battery can be realized with a small amount of the metal cobalt or the cobalt compound. Besides, since the formed conductive matrix is firmer than the conductive matrix formed at the time of the initial charging in a room temperature, an effect of hardly causing the decline of the capacity even after leaving can be achieved.
However, a problem is involved in that a high temperature atmosphere is required as the initial charging environment, and further, a longer initial charging time is needed so that the production cost as a whole is increased.
Furthermore, the official gazette of Japanese Patent Publication Laid-Open No. 9-73900 discloses a method of precipitating cobalt hydroxide on the surface of nickel hydroxide particles, fluidizing or dispersing the same in an open device of a hot air convection method, spraying an alkaline aqueous solution thereto and agitating the same in a hot air flow such as hot air so as to convert the above-mentioned cobalt hydroxide into an oxide of higher order.
Since an oxide of higher order of the cobalt is already formed on the surface of the nickel hydroxide particles produced by the method, itself is an active material with an improved utilization. A battery with the nickel hydroxide particles assembled as the positive electrode active material has the above-mentioned conductive matrix formed already at the time of completing the assembly.
However, the method disclosed in the official gazette of Japanese Patent Publication Laid-Open No. 9-73900 has the following problems.
The first problem is that the particles prepared by making the cobalt compound precipitate on the surface of the nickel hydroxide particles should be used as the starting material. In order to make the cobalt compound precipitate, as disclosed in the embodiments of the above-mentioned prior art, complicated procedure is required in terms of the concentration of the chemicals to be used the pH adjustment of the reaction field, control of the reaction time, and the like, and thus the conditions requiring a higher cost are needed so that it is problematic industrially.
Moreover, since the heat treatment is conducted in a hot air convection method, the heat efficiency is poor, and thus it is another factor to raise the production cost. Besides, since an open system is required as the field for converting cobalt hydroxide into an oxide of higher order in a hot air convention method, a problem is involved in that an alkaline aqueous solution can easily be evaporated. With the evaporation of the alkaline aqueous solution, the amount of the cobalt compound precipitated on the surface of the nickel hydroxide particles, dissolved in the alkaline aqueous solution is reduced, and thus it disturbs the re-precipitation of the dissolved cobalt compound as an oxide of higher order.
As an actual problem, in a battery assembled with a nickel electrode provided using an active material produced in the method, the utilization of the active material is low, and furthermore, a problem of decline of recovering the capacity after the storage in a high temperature environment or after a long term storage is caused.
Moreover, in order to improve the heat efficiency at the time of reaction in the above-mentioned method, the flow amount of the hot air can be increased, but such a measure cannot be adopted since it will promote evaporation of the alkaline aqueous solution.
As heretofore mentioned, in the above-mentioned prior arts, although nickel hydroxide particles having a factor of a conductive matrix for improving the utilization of the active material when used in a nickel electrode already formed can be produced, the above-mentioned various problems are involved.